only strip continuation prompts if regular prompts seen first

should be discussed before merge (ping @takluyver),
as it is a significant and sensitive change.

I'm also not sure that it's the right fix, but worth a try.

closes #4059
store default codemirror mode in only 1 place
do not include specific css in embedable one
use unicode for HTML export

not utf8-encoded bytes

should close #3807
fix InlineBackend config message

had wrong class name, was unclear.
catch if cm_config of other key undefined
added new readme in the directory
mention pr folder in whatsnew and CONTRIBUTING
adding a couple of examples

since these are empty files, the eventual tools/ script that will
concatenate these into development.txt can use these as a test case
fix links
apply @takluyver's suggestion
added bolding of "must have"
cleanup Client.close

allows setting `linger` on sockets at shutdown time.
close Client sockets if connection fails

avoids runaway FDs in cases of frequent client failure.
Implement a better check for hidden values for %who etc.
Tweaks to interactiveshell for user_ns_hidden as dict
FIXED, nbconvert sphinx preprocessor date format string for Windows
Adding citation support.
FIX, coalescestreams incorrect nesting.
Adding better logic to the PDF postprocessor.